The invention relates to a receiver for signals transmitted with a carrier frequency, comprising a baseband conversion circuit and an arrangement for estimating a frequency deviation.
With baseband conversion of signals transmitted with a carrier frequency, signals received in the receiver are convened to the baseband frequency after they have been convened as appropriate to one or more intermediate frequencies, for example, by means of a quadrature mixer. Tolerances and drift of the transmitter frequency and of the mixing frequencies in the receiver may lead to a frequency deviation in the baseband signal, so that the transmission performance is affected in a negative fashion. In addition, Doppler effects in radio transmission systems comprising mobile transmit and/or receive arrangements make a relatively rapidly varying frequency deviation possible, which is additively superimposed on the afore-mentioned frequency deviation which varies only very slowly.
In order to avoid or restrict a negative effect on the transmission performance, which effect is caused by a frequency deviation in the receiver, it is necessary to continuously estimate the frequency deviation and keep this deviation as small as possible by means of an automatic mixing frequency control, or equalize this deviation by means of various analog or digital signal processing measures, for example, a frequency correction in the digitized baseband signal.
The article by F. D. Natali, "AFC Tracking Algorithms", in IEEE Transactions on Communications, Vol. COM-32, August 1984, pp. 935 to 947, discusses various circuit arrangements for estimating a frequency deviation for receivers having baseband conversion. These frequency deviation estimators are suitable for many digital modulation methods, but will operate satisfactorily only when the transmit channel is free from distortion or causes only little signal distortion. With strongly dispersive transmit channels, as they may occur in both radio transmission and line-bound transmission systems, the results obtained with these frequency deviation estimators are no longer satisfactory.